JERSEY CITY — When Charles Gwathmey died in the summer of 2009, New York City lost one of its most prolific and influential architects. Adherents of the strict rationalism of Modernist design, Mr. Gwathmey and his partner, Robert Siegel, had still managed to infuse their clean lines and grand geometries with warmth and humanity in more than 400 projects spanning four decades, including the expansion of the Guggenheim Museum and a new building for the United States Mission to the United Nations.

For Mr. Siegel, it was almost as if he had lost a part of himself.

“For 42 years, we sat across the desk from each other, we sketched, we drew, we talked, we argued, we worked,” Mr. Siegel said last week inside his Battery Park City apartment. “It became difficult to pretend you’re just going to continue on as it was before.”

Anyone looking at 70 Columbus, a 50-story apartment tower that opened here in November, might think Mr. Gwathmey was still seated across from his old partner, swapping ideas for the building’s unconventional trapezoidal layouts and its 545 apartments, the cascading courtyard, its doorknobs and countertops. It is a continuation of what came before, but also a coda to the Gwathmey-Siegel legacy, one Mr. Siegel has had to maintain alone.